/Engine/Source/Runtime/Slate/Public/Framework/Application/SlateApplication.h
Locates the SlateUser object corresponding to the index, creating a new one if it doesn't exist. Asserts if given an invalid (ie, negative) index.
[TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [FSlateUser](API\Runtime\Slate\Framework\Application\FSlateUser) > GetOrCreateUser ( [FPlatformUserId](API\Runtime\Core\Misc\FPlatformUserId) PlatformUserId )